A photodetector disclosed in Japanese Published Patent Application 2-9180 is Patent described as follows.
FIG. 5(a) shows a plan view of the above-described prior art photodetector and FIG. 5(b) shows a crosssectional view taken along line A--A' of FIG. 5(a). In FIG. 5(a), reference numeral 1 designates a high resistance and semi-insulating substrate comprising CdTe. Reference numeral 11 designates a first semiconductor layer comprising p type Cd.sub.0.3 Hg.sub.0.7 Te. Reference numeral 12 designates a second semiconductor layer comprising p type Cd.sub.0.2 Hg.sub.0.8 Te. Reference numeral 13 designates an n type impurity doped layer. Reference numeral 14 designates a pn junction in the first semiconductor layer 11. Reference numeral 15 designates an n type impurity doped layer. Reference numeral 16 designates a pn junction at the second semiconductor layer. Reference numeral 17 designates a common p side electrode. Reference numeral 18 designates an N side electrode. Reference numeral 19 designates an n side electrode.
FIGS. 6(a)-6(c) show energy band diagrams of a heterojunction in the infrared detecting element of the photodetector of FIG. 5(a). FIG. 6(a) shows an energy band diagram in the area of the n type impurity doped layer 15 and the second semiconductor layer 12, and FIG. 6(b) shows an energy band diagram in the area of the n type impurity doped layer 15, the second semiconductor layer 12, the first semiconductor layer 11, and the second semiconductor layer 12. FIG. 6(c) shows an energy band diagram in the area of the N type impurity doped layer 13, the first semiconductor layer 11, and the second semiconductor layer 12.
Among the infrared rays incident to the infrared detector element of such a structure, those of 3 to 5 microns are absorbed by the first semiconductor layer 11 of wide energy band gap, and those of 10 microns wavelength transmit the first semiconductor layer 11 and are absorbed by the second semiconductor layer 12 of narrower energy band gap. Carriers exited by the infrared rays of 3 to 5 microns in wavelength and absorbed by the first semiconductor layer 11 below the n side electrode 19 are detected between the n side electrode 19 and the common p side electrode 17 after following the path shown in FIG. 6(b). Carriers exited by the infrared rays of 10 microns wavelength and absorbed by the second semiconductor layer 12 below the n side electrode 19 are detected at a region between the n side electrode 19 and the common p side electrode 17 after following the path shown in FIG. 6(a). Carriers exited by the infrared rays of 3 to 5 microns wavelength and absorbed by the first semiconductor layer 11 below the N side electrode 18 are detected at a region between the N side electrode 18 and the common p side electrode 17 after following the path shown in FIG. 6(c). The infrared rays of 10 microns wavelength incident to below the N side electrode 18 transmit the first semiconductor layer 11 and are not detected.
From the above, at below the n side electrode 19, the infrared rays of 10 and 3 to 5 micron wavelengths are detected and only those of 3 to 5 microns wavelength are detected below the N side electrode 18.
Such a photodetector device is combined with silicon charge coupled devices to produce a hybrid type solid state imaging device and is utilized in an infrared image tracking apparatus. By way, as a measure to avoid infrared image tracking, there is a method of using a high temperature material as a decoy. In FIG. 7, reference numeral 9 designates a target and reference numeral 10 designates a material other than the target which is used as decoy.
FIG. 8 is a diagram showing the dispersion of radiation of black body which is calculated from the Planck's law. From this figure, it is understood that both in the neighborhood of room temperature (about 300.degree. K) and at high temperature (of about 1000.degree. K) material emits infrared rays of 10 microns wavelength. Therefore, when an infrared detector element responsive to 10 microns wavelengths of Cd.sub.0.2 Hg.sub.0.8 Te is used, it is impossible to distinguish the target 9 from the other material 10. Herein, as is understood from FIG. 8, the ratio of the infrared intensity between the 5 micron wavelength and 10 micron wavelength is 1:3 for a target of 300.degree. K, but the ratio of the infrared intensity between the 5 micron wavelength and 10 micron wavelength is 10:1 when the other material is at 1000.degree. K. Therefore it is possible to distinguish the target 9 and the other material 10 by taking the ratio of the output signal of the infrared detector element in the 3 to 5 microns wavelength band and that at the 10 micron wavelength.
In the prior art photodetector device, because the first semiconductor layer 11 and the second semiconductor layer 12 need be produced, a higher level of crystalline growth technique than if only one layer is grown is required. Furthermore, in order to enhance the degree of integration of pixels in the above-described construction, a film thickness of above 10 microns is required to improve the absorption coefficient sufficiently while giving consideration to the absorption coefficient (10.sup.3 cm.sup.-1) of the second semiconductor layer for the light of 10 microns wavelength where respective pixels are produced in 50 micron pitches. Therefore, the production of the device is impossible using only the fabrication techniques which are present applicable to the semiconductor devices.